A data processing system typically comprises a memory for storing computer program code instructions and a central processing unit (CPU) for executing the computer program instructions. In operation, the memory also stores input data to be operated upon by the computer program code and output data produced by execution of the computer program code. In general, the computer program code can be divided into operating system code and application program code. The operating system code configures the CPU for executing the application program code. Conventionally, the memory is implemented by a combination of solid state memory such as random access memory and rotating disc mass data storage such as magnetic or optical disc storage.
A recent addition to the field of data storage technology is generally referred to as local probe storage technology. As described in Vettiger et al. “The Millipede”—More than one thousand tips for future AFM data storage, P. Vettiger et al, IBM Journal of Research and Development. Vol. 44 No. 3, May 2000, a local probe storage array typically comprises a storage surface having a locally deformable film disposed thereon and an array of micro mechanical probe sensors. Each probe sensor has a probe tip of nanometer scale dimensions facing the deformable film. In operation, during a data write operation, the array tips are brought into proximity to the storage surface. Energy is selectively applied to each tip, typically in the form of heating and force. The energy applied to the tips is transferred to the storage surface. The energy transfer produces a local deformation in the storage surface in the vicinity of each energized tip. The array of tips is moved to a new location on the storage surface between successive write operations. This is in preparation for writing to new locations on the storage surface. During a read operation, the array tips are scanned relative to the storage surface. Local deformations of the storage surface produced during the aforementioned write operation produce deflections in the tips as they are scanned over the surface. Such deflections can be detected thermally or optically. The presence or absence of a local deformation in the storage surface can be detected by an atomic force microscopy (“AFM”) tip as described above as a stored “1” or stored “0”.
An improved local probe storage technology described in the commonly assigned, copending patent application, Data Processing System, EP Application No. 02405643.4, filed 23 Jul. 2002 of Gerd K. Binnig et al., provides a data processing system having a local probe storage array with a plurality of sensors for reading data from a storage surface; a plurality of data processing elements mounted on the storage array and each connected to a different sensor of the array for processing data read by said connected sensor. The data processing elements may be logic gates for performing simple comparisons with input data. Alternatively, each data processing element may comprise complex logic circuitry for performing more complex functions based on data read by the storage array.
The storage surface preferably has a plurality of data fields with each data field corresponding to a different sensor and each data field having a matrix of bit storage locations individually addressable by the corresponding sensor for writing, reading, and erasing data.
This results in a local probe array-based storage device and data processing method. The method further comprises writing data to a storage surface, reading data from a storage surface, and erasing data from a storage surface by locally deforming a storage surface and by reading the deformations, all via sensors of a local probe storage array; and processing the data read from the surface via a plurality of data processing elements mounted on the storage array and each connected to a corresponding sensor of the array.
This further results in a memory for storing data where the memory comprises a local probe storage array having a plurality of sensors for reading data from a storage surface. The storage surface comprises a plurality of data fields each corresponding to different sensors and each having a matrix of bit storage locations individually addressable by the corresponding sensor. Each field of the storage surface has different bit locations assigned to different memory pages.
The read-write mechanism in the device (referred to herein as an atomic force microscopy device) relies on thermomechanical deformation of a thin polymer film. As should be obvious, control of the heat and load transfer characteristics are highly critical aspects of the polymer and polymer film characteristics. Some control over heat and load transfer can be engineered into the design of the device's cantilever arrays. However, thermal and mechanical control must also be conferred by thoughtful design and fabrication of the polymer structure. Careful choice of the synthetic components can lead to a polymeric material with the modulus of elasticity and the thermal conductivity optimized for device energy efficiency, storage density, and long-term stability. The design of these polymeric recording layers relates to the ultra high density data storage systems of the type in which a tip comes into proximity with the polymer layer in order to execute bit-writing and bit-reading. In these data storage designs, information is detected by scanning the surface of the medium with a tip positioned in full or partial contact with, or in proximity to the polymeric recording layer and data bit values are determined by the topographical state or thermal transfer at the bit location.
The specific sensor technology is based on the atomic force microscope, a well-known device in which the topography of a sample is sensed by a tip mounted on the end of a microfabricated cantilever. As the sample is scanned, the interaction between the tip and the sample surface causes pivotal deflection of the cantilever. The sample topography is determined by detecting this deflection. More recently, this AFM technology has been applied to the field of data storage with a view to providing a new generation of high-density, high data-rate storage devices for mass-memory applications.
The cantilever-mounted tip, referred to as the read/write tip, is used for reading and writing of data on the surface of a data storage medium. In operation, the read/write tip is placed in proximity to the surface of the data storage medium. Heretofore, the storage medium typically comprised a linear polymeric material. In the write mode, the read/write tip is heated which results in heat transfer to the polymer surface layer causing local softening of the linear polymer. This allows the tip to penetrate the surface layer to form a pit, or bit indentation; such a pit may represent a bit of value “1”, a bit of value “0” being represented by the absence of a pit. Alternatively, a pit may represent a bit value of “0”, with a bit value of “1” being represented by the absence of a pit. This technique is referred to as thermomechanical writing.
The storage medium can be moved relative to the read/write component tip allowing the tip to write data over an area of the surface, or “storage field”, corresponding to the field of movement (surface area laterally accessible to the tip). Each indentation is created by heating the cantilever tip and with the application of force moving this tip toward the polymer film. The tip is heated by passing a current through a resistive heater associated with the cantilever. Some of the heat generated in the resistor is conducted through the tip and into the polymer film, locally heating the polymer. If sufficient heat is transferred to raise the temperature of the polymer above a certain temperature (which is dependent on the chosen polymer), the linear polymer softens or goes through its glass transition temperature and the tip sinks in, creating an indentation or bit.
In the read mode, the storage field is scanned by the tip, the position of the tip and the cantilever on which it is mounted topographically tracking the presence or absence of a pit. The reading operation uses thermal sensing based on the principle that the thermal conductance between the cantilever, and components attached thereto, and the storage substrate, changes according to the distance between them; the distance is reduced as the tip moves into a bit indentation.
Heretofore the storage medium consisted of a bulk linear polymer layer, optionally implemented as a silicon substrate having a very thin layer of linear polymethylmethacrylate (PMMA) as the read/write layer. The advantage of having a silicon substrate is that the hard silicon substrate limits the penetration of the tip (this feature can also be a disadvantage) and also, because silicon is a better conductor of heat than polymers such as PMMA, there is improved transport of heat away from the pits during the reading and writing processes. The linear PMMA layer is preferably about 40 nm thick thus giving a depth of pit of up to 40 nm.
Some problems of tip wear are believed to be caused by the tip penetrating the polymer layer and making contact with the hard silicon substrate, and in a still further improved storage medium, a layer of crosslinked material, for example, SU-8 resin from MicroChem Corporation, Newton, Mass., USA, was introduced between the PMMA and the silicon substrate. SU-8 is an octafunctional epoxy resin crosslinked by a cationic photoinitiator. The layer of crosslinked material, typically about 70 nm thickness, acts as a softer penetration stop thereby reducing tip wear. When the cross-linked material is present between the linear polymer layer and the silicon substrate, no penetration into the crosslinked SU-8 layers is observed and the cross linked layer does not act as the recording layer in this example.
The efficiency of writing and reading the indentations (bits of information) is critically dependent on the nature of the polymeric thin film. Desirable attributes of the polymeric thin film are ‘softness’ and deformability during the writing phase, toughness/resistance to wear during the reading phase, and long-term stability over the lifetime of the stored data. A hard polymer with a high melting point will be difficult to soften sufficiently for the tip to sink in and form the pit during the writing process. By way of contrast, a hard polymer will be preferred during the reading process since the tip is required to travel across the polymer surface and the surface must be sufficiently hard and smooth to minimize the wear on the tip and damage to the surface. Finding a material with these properties is desirable and challenging since one feature normally precludes the other. Linear polymers such as PMMA have been found to have suitable writing temperatures and the force required on the tip to form the pit is acceptably low for the required tip performance and power consumption; however, the wear rate on reading has been found to be unacceptably high because of the softness of the surface. Specifically, a recording layer of linear PMMA erodes at the rate of about 0.01 nm per scan. This is an unacceptably high wear rate.
There is a clear need to overcome these problems by using a class of polymers that combine low writing temperatures, ready pit-forming by the probe for writing, and a low wear rate.